Renamed the tmx MIME type to application/x-tiled-tmx

This should make it easier to add further MIME types of plugins.

The icon is not renamed to application-x-tiled-tmx in order to allow
other plugins to depend on it in future too.
Small tweaks to the desktop file

The TryExec line makes sure that tiled is only listed if the executable
exists. Meaning that the desktopfile should not be displayed as an menu
entry if the executable is missing in $PATH (for example if you install
with a custom prefix).

Setting the GenericName might have some use for some window managers /
application menus (I was unable to find this displayed anywhere in the
gnome-panel manu, but it cannot hurt).